This invention is in the field of laser control, and is more specifically directed to the control of laser power in a selective laser sintering system.
The field of rapid prototyping of parts has, in recent years, made significant improvements in providing high strength, high density, parts for use in the design and pilot production of many useful articles. "Rapid prototyping" generally refers to the manufacture of articles directly from computer-aided-design (CAD) data bases in an automated fashion, rather than by conventional machining of prototype articles according to engineering drawings. As a result, the time required to produce prototype parts from engineering designs has reduced from several weeks to a matter of a few hours.
One example of a rapid prototyping technology is the selective laser sintering process practiced by systems available from DTM Corporation of Austin, Tex. According to this technology, articles are produced from a laser-fusible powder in layerwise fashion. According to this process, a thin layer of powder is dispensed and then fused, or sintered, by laser energy that is directed to those portions of the powder corresponding to a cross-section of the article. After the selective fusing of powder in a layer, an additional layer of powder is then dispensed, and the process repeated, with fused portions of later layers fusing to fused portions of previous layers (as appropriate for the article), until the article is complete. Detailed description of the selective laser sintering technology may be found in U.S. Pat. No. 4,863,538, U.S. Pat. No. 5,017,753, U.S. Pat. No. 5,076,869, and U.S. Pat. No. 4,944,817, all assigned to Board of Regents, The University of Texas System, and in U.S. Pat. No. 4,247,508 assigned to DTM Corporation, all incorporated herein by this reference. The selective laser sintering technology has enabled the direct manufacture of three-dimensional articles of high resolution and dimensional accuracy from a variety of materials including wax, polycarbonate, nylon, other plastics, and composite materials such as polymer coated metals and ceramics. Wax parts may be used in the generation of tooling by way of the well-known "lost wax" process.
Conventional selective laser sintering systems, such as the SINTERSTATION 2000 system available from DTM Corporation, position the laser beam by way of galvanometer-driven mirrors that deflect the laser beam. The deflection of the laser beam is controlled in combination with modulation of the laser itself, to direct laser energy to those locations of the fusible powder layer corresponding to the cross-section of the article to be formed in that layer. The laser may be scanned across the powder in raster fashion, with modulation of the laser effected in combination therewith, or the laser may be directed in vector fashion. In some applications, cross-sections of articles are formed in a powder layer by fusing powder along the outline of the cross-section in vector fashion either before or after a raster scan that "fills" the area within the vector-drawn outline.
The thermal fusing mechanism in the selective laser sintering process depends upon the laser energy flux density, which is the thermal energy received to each location of powder per unit time. The laser flux density depends upon the laser energy, the laser spot size, and the temporal duration of the exposure of the powder to the laser energy. For most materials, the shape and density of the article being formed is sensitive to the laser energy flux density, such that even slight deviations in the laser energy flux density result in less than ideal article attributes. For example, if the laser energy flux density is too low, the article will be mechanically weak; conversely, excessive laser energy flux density can result in poor fidelity of the article dimensions relative to the CAD representation, overheating of the powder, or even burning of the powder.
The effects of geometry-dependent non-uniformity of laser energy flux density have been observed in articles fabricated by selective laser sintering, especially in the sensitive materials noted above. Typically, these effects are evident at the terminal ends of raster or vector scans. For example, conventional selective laser sintering systems simply apply a constant power level to the laser, gating the laser on and off to selectively fuse the powder. While the laser responds quite rapidly to this gating action, the galvanometers that direct the aim of the laser beam do not instantaneously accelerate and decelerate. Accordingly, using constant laser output power, the laser energy flux density received by the powder during acceleration and deceleration of the galvanometers, typically at the beginning and end of each scan, is greater than that applied to the powder once the galvanometers have reached their full scan speed. The effect of this increased laser energy is referred to in the art as "end-of-vector blooming", as the higher laser energy flux density due to increased dwell time at the ends of scan vectors will typically cause the fusing of the powder to spread, or "bloom", laterally outside of the desired scan path. This blooming is evident in the finished article as poor cosmetic appearance and, if excessive, as dimensional inaccuracy.
A known approach to eliminate end-of-vector blooming inserts programmable timing delays between the initiation of a scan and the gating on of the laser, to allow the galvanometers to at least partially accelerate prior to the application of laser power; at the end of a scan, laser power can be gated off prior to decelerating the galvanometers. However, particularly at the beginning of the scan, improper selection of the delay can be quite difficult, especially considering that the optimal delay time to avoid dimensional error can only be selected for one set of conditions (i.e., laser power and scan speed). Furthermore, a tradeoff also exists between avoidance of end-of-vector blooming and feature resolution, considering that long delays between initiation of a scan and gating of the laser reduces the resolution with which fine features in the article can be fabricated, as is especially evident in thin walled articles. In addition, the optimum delay time can dynamically vary within the build of a single article, as delay time optimization depends upon scan vector lengths, and also upon the distances between scanned vectors (i.e., the lengths of "jumps"). Besides being time-consuming and difficult, the proper choice of delay time is also often dependent upon the dynamic behavior of individual galvanometers, and thus may vary from system to system.
By way of further background, U.S. Pat. No. 5,352,405, issued Oct. 4, 1994 assigned to DTM Corporation, and incorporated herein by this reference, describes a method of scanning the laser across the powder in a selective laser sintering apparatus to provide a uniform time-to-return of the laser for adjacent scans of the same region of powder, thus providing uniform thermal conditions over the cross-section of the article. As described therein, this method scans from only slightly outside of the cross-section of the article to be formed, so that the delay between the start and stop positions of the scan and the times at which the laser is on are relatively small. Time-to-return of the laser from scan-to-scan is made more uniform, and the overall scan time for the build cycle is reduced. However, the close relationship between the scan limits and the times at which the laser is on cause the laser to be turned on during acceleration and deceleration intervals at the ends of the scans, during which more energy is delivered to the powder per unit time than when the scan is at full speed. Distortion of the article being formed can thus result.
By way of further background, U.S. Pat. No. 5,014,207, issued May 7, 1991, describes a technique for modulating the laser power in accordance with sweep speed in a stereolithography system. As is well-known in the art, stereolithography refers to a technology by way of which three-dimensional articles are formed by the selective application of laser energy to successive layers of a photopolymerizable liquid. As is described in detail in U.S. Pat. No. 5,014,207, the formation of articles by stereolithography had previously been vulnerable to variations in the depth of hardening of the photopolymerizable liquid. Specifically, this reference discloses that the cross-section of solidification of a scan of the photopolymerizable liquid is nonGaussian. Because of this effect, undesirable ripple would be present in the lower surface of the article being formed, unless the non-Gaussian profile is taken into account by closely spacing adjacent scan lines; this close spacing requires increased scan velocities in order to maintain reasonable build times. The reference further discloses that the use of high scan velocities in turn resulted in longer acceleration times, which caused non-uniform depth of solidification along scan lines due to non-uniformities in scan velocity during the acceleration times. The reference discloses a technique of controlling laser power (i.e., duty cycle) in accordance with scan velocity, so that the laser beam is turned on for a set period of time per unit distance over the full scan.
It has been observed, however, in connection with the present invention, that the depth of hardening of powder by selective laser sintering does not substantially depend upon the laser power density applied to the powder. This insensitivity of depth of hardening is due to the thermal nature of the selective laser sintering mechanism, which in fact generally follows a Gaussian profile. Accordingly, variations in dwell time of a constant power laser beam over powder, such as during acceleration portions of a scan, does not result in variations of the depth of hardening, but instead is manifest in lateral blooming of the hardened structure outside of the laser scan line because of conduction of the thermal energy in the powder away from the scanned locations . In contrast, no lateral blooming is evidenced in stereolithography, as photopolymerization occurs only at locations at which photons impinge upon the liquid, with no conduction occurring outside of the scan lines.